absorption-refrigeration

name	absorption-refrigeration
description	Absorption refrigeration — LiBr-water and NH3-water cycles, COP, generator/absorber/condenser/evaporator heat duties, waste heat driven cooling, double-effect cycles, ASHRAE standards.
metadata	{"priority":7,"promptSignals":{"phrases":["absorption refrigeration","absorption chiller","LiBr water cycle","ammonia water refrigeration","waste heat cooling","absorption cooling"],"minScore":3}}

Absorption Refrigeration — Complete Skill

Cycle Overview

Absorption cycle replaces compressor with: Generator (heat input) + Absorber (heat rejection) + Solution pump (low work) Working pairs: LiBr-H₂O (water as refrigerant; LiBr as absorbent) or NH₃-H₂O (NH₃ as refrigerant)

Component functions:

• Generator: heat Q_gen desorbs refrigerant vapor from strong solution
• Condenser: refrigerant vapor condenses at high P (rejects Q_cond)
• Expansion valve: throttles refrigerant to evaporator pressure
• Evaporator: refrigerant evaporates, absorbing Q_evap (cooling effect)
• Absorber: weak solution absorbs refrigerant vapor; rejects Q_abs
• Solution pump: pumps weak solution from low P to high P (small W_pump)
• Solution HX: recovers heat between strong→absorber and weak→generator streams

Energy Balance

For cycle at steady state: Q_gen + Q_evap + W_pump = Q_cond + Q_abs

Coefficient of Performance (COP): COP = Q_evap / (Q_gen + W_pump) ≈ Q_evap / Q_gen [W_pump << Q_gen; neglected]

Theoretical maximum COP (Carnot-limited): COP_max = (T_evap / (T_gen - T_evap)) × ((T_gen - T_cond) / T_cond) T_evap = evaporator temperature [K]; T_gen = generator temperature [K]; T_cond = condenser/absorber T [K]

Typical single-effect COP: LiBr-H₂O: COP = 0.65–0.80 (generator T = 80–100°C; condenser T = 35°C; chilled water 7°C) NH₃-H₂O: COP = 0.50–0.65 (lower due to rectifier losses and refrigerant impurity)

LiBr-H₂O Cycle

Refrigerant: water (H₂O); excellent refrigerant properties; zero ODP/GWP Absorbent: lithium bromide (LiBr); highly hygroscopic salt Operating pressure: low side 0.8–1.0 kPa (0.006–0.007 bar) at 5°C evaporator; high side 7–8 kPa

Concentration: Strong solution (leaving generator): 62–65% LiBr by mass Weak solution (leaving absorber): 58–60% LiBr by mass ΔX = 3–6% per cycle

Solution crystallization risk: Crystallization occurs if solution concentration too high at low T → plugging Crystallization line: X > 70% at 30°C; maintain safety margin Purge/vacuum required: non-condensable gases (N₂, O₂) must be removed continuously

Heat duties: Q_gen = m_r × (h_rv - h_sl) + m_ss × (h_ss - h_ws) [kW; m_r = refrigerant flow; h = enthalpy] Q_evap = m_r × (h_rv_evap - h_cond_exit) [kW] COP = Q_evap / Q_gen

Typical T ranges: Generator supply: 70–95°C hot water (solar, waste heat, steam) Condenser/absorber: 25–40°C cooling water Evaporator: 5–12°C chilled water outlet

NH₃-H₂O Cycle

Advantage over LiBr: works below 0°C (refrigerant NH₃ freezes water above 0°C limit of LiBr systems); suitable for industrial refrigeration and heat pumps Complication: H₂O vapor pressure non-negligible → requires rectifier to purify NH₃ vapor

Rectifier duty: Q_rect = m_r × (h_vapor_out - h_rectified) [kW; reduces Q_evap effectiveness]

Typical generator T: 120–180°C (steam at 150–200 kPa or direct fired) Evaporator T: -40°C to +10°C (industrial cold storage: -30°C) COP: 0.45–0.65 (single effect); 0.8–1.1 (double effect)

Double-Effect LiBr Cycle

Configuration: two generators at high and low pressure; condenser heat from first effect drives second generator COP improvement: ~1.2–1.4 (vs. 0.7 single effect) Required generator T: 150–175°C (steam at 400–800 kPa) First generator: T_gen1 = 140–160°C (high pressure) Second generator: T_gen2 = 70–90°C (uses condenser heat from first generator)

COP (double-effect): COP ≈ 2 × COP_single_effect × η_regeneration ≈ 1.2–1.4 Q_saved = extra refrigerant from second generator with same Q_gen input

Triple-Effect Cycle

Three generators; COP ≈ 1.6–1.8; generator T 200–230°C; commercially available in large sizes

Equipment Sizing

Chiller capacity: Q_chiller = m_chw × c_p × (T_in - T_out) [kW; m_chw = chilled water mass flow; c_p = 4.18 kJ/(kg·K)] Typical chilled water ΔT = 5–7°C

Generator heating coil: Q_gen = Q_evap / COP [kW] Steam consumption: m_steam = Q_gen / h_fg [kg/s; h_fg = 2200 kJ/kg at 100°C]

Cooling tower load: Q_CT = Q_evap + Q_gen [kW] (total heat rejection) Cooling water flow: m_cw = Q_CT / (c_p × ΔT_cw) [kg/s; ΔT_cw = 5–8°C]

Absorber sizing: Q_abs ≈ Q_evap + Q_gen - Q_cond (from heat balance) Heat transfer area: A = Q_abs / (U × LMTD) [m²; U ≈ 1.0–1.5 kW/(m²·K) for falling film]

Applications

Application	Working pair	COP	Heat source
Building HVAC	LiBr-H₂O	0.7	Hot water 85°C
Solar cooling	LiBr-H₂O	0.7	Solar flat plate
Industrial cold	NH₃-H₂O	0.5	Steam 150°C
Cogeneration	LiBr-H₂O DE	1.3	Steam 175°C
Waste heat recovery	LiBr-H₂O	0.7	Flue gas HX

Standards

Standard	Scope
ASHRAE 147	Absorption chillers; performance testing
ARI 560	Absorption water chilling packages rating
ASHRAE 90.1	Energy efficiency limits for chillers
ISO 5149	Refrigerating systems and heat pumps

Output

Provide: working pair (LiBr-H₂O/NH₃-H₂O), cycle type (single/double/triple effect), COP at operating conditions, heat duties Q_gen/Q_evap/Q_cond/Q_abs [kW], generator supply temperature [°C] and heat source type, cooling capacity [kW or TR], chilled water supply/return temperatures [°C], cooling water flow and temperature [°C], steam or hot water consumption rate [kg/s or kW], crystallization safety check (LiBr concentration vs. limit), applicable standard (ASHRAE 147, ARI 560).